1. Technical Field
The present invention generally relates to alkali metal thermal to electric conversion (AMTEC) cells and, more particularly, to an AMTEC cell including radially projecting beta aluminum-type solid electrolyte elements and a central heat input region.
2. Discussion
An AMTEC cell is a thermally regenerative concentration cell typically utilizing an alkali metal, such as sodium or potassium, as a working fluid and a solid electrolyte as an ion selective membrane. While sodium is referred to throughout this description as the working fluid, it is to be understood that other alkali metals are also applicable to this invention. The electrolyte separates the cell into a high pressure zone and a low pressure zone while permitting a nearly isothermal expansion of the alkali metal for generating high-current/low voltage power at high efficiency. To accomplish this, AMTEC cells commonly employ at least one beta-alumina type solid electrolyte (BASE) element, typically a tube, which is exposed to high-pressure sodium on a first surface and low-pressure sodium on an opposite surface. The BASE element's opposed surfaces are overlaid with permeable electrodes which are connected to each other through an external load circuit.
In operation, neutral sodium atoms incident on one surface of the BASE element release their electrons to one electrode (the anode). The resulting sodium ions pass through the element wall under the applied pressure differential and the emerging sodium ions are neutralized at the other electrode (the cathode) by electrons returning from the external load. As such, the pressure differential drives the sodium through the BASE element thereby creating an electrical current which passes through the external load resistance.
The neutral sodium vapor at the outer electrodes migrates through the chamber between the BASE element and the cell wall until it condenses at a low-temperature condenser at the cold end of the cell. From there, the sodium condensate flows through an artery containing a fine pore membrane commonly consisting of a packed metallic felt toward the high pressure zone of the cell. The liquid sodium evaporates at a high temperature evaporator which is coupled to the artery and is returned to the inside of the BASE element through a common plenum at the opposite "hot" end of the cell.
Early generation AMTEC cells include a single BASE element coaxially located within the interior volume of a cylindrically shaped cell wall. Later generation cells employ multiple BASE elements electrically coupled in series and aligned concentrically about and parallel to a longitudinal axis of a cylindrically shaped cell wall. Although these conventional cells have been quite successful, there is room for improvement in the art. For instance, it would be desirable to provide an AMTEC cell including many more BASE elements than according to the prior art while minimizing the amount of cell wall surface area available for parasitic heat transfer. It would also be desirable to provide an AMTEC cell having a single, centrally located, heat input region for conveying energy for heating the cell from a common heat source. Additionally, it would also be desirable to provide an expanding area between the BASE elements for alkali metal vapor flow. Furthermore, it would be desirable to provide a convenient platform for supporting significantly more BASE elements than according to the prior art.